Before I try my solar chimney idea, I will try something else I noticed years ago when I carried a hard dinghy over our forward hatch. Without the dinghy over the hatch, the v-berth would be too hot to use at all but with the dinghy making a large enclosed area over the hatch open at the lower edges, somehow it produced a huge amount of airflow through the v-berth with any forward motion.
So, I'll probably try making a tent like thing to fit over the forward hatch with a lot of open area ta the bottom. Should be easy, use some 3/16 fiberglass poles from McMaster Carr and make the tent thing from Tyvek. Fasten it to the toe rails.

Did the dinghy actually improve airflow or did it cool by means of shading the deck? Before you go to a lot of trouble to construct some sort of tent, I'd suggest an experiment with cheap materials.

Materials you will need:
-- Two air thermometers
-- A cheap, light-colored tarpaulin
-- Around 10 tarp clips.
-- Some cheap line (e.g. clothesline)

1) Place a thermometer out of the sun in the V berth in late morning on a hot, sunny day and open the hatch, Place the other outside the boat in shade.

2) Wait a half hour or so for the v-berth temperature to stabilize.

3) Take thermometer readings and record them along with the time.

4) Tie the tarp about bow pulpit height to your shrouds and forestay. Maybe tie it off to your bow pulpit as well to increase the width near the forepeak. Use the extra tarp clips to fold the loodr flaps up on top of the resulting surface and clip them in place so they don't hang down and don't blow around.

5) Wait an hour for the temperature to stabilize in the v berth and then read the thermometers again. Record the temperatures and the time.

6) Compute the two inside/outside temperature differences. Did the tarp help, or did it help enough to matter? In addition to noting the improvement, if any, how does the V berth feel subjectively? Is it more comfortable now? Keep in mind that the difference will become greater as you move south and Sun becomes stronger (i.e. the level of insolation will increase).

If the bow tent works for you, trim the cheap tarp to shape and use it as a template to cut canvas (in the long term, you won't like the rattly noise tarps make and neither will your neighbors) for your tent. I exepct you will also want to add some kind of pole in the center to keep the canvas from sagging and catching rainwater.

For the main cabin, also consider a boom tent. I experimented last Summer with a cheap tarp that covered the entire boom -- from the mast all the way to my bimini and it was great! I tied it off to my shrouds and the bimini framework. This year I am going to make one of canvas.

Obviously, neither tent can be used with sails deployed but they're great at anchor or tied up.

i use plastic tarps in a layered fashion so as to bring into the boat air and prevent su n from baking inside and decking. works great and has worked great for many many years. no , isnt gorgeous, but keeps boat interior 10 degrees cooler than other boats. when it hits 110 in daylight i turn on air conditioner. i only use the lower settingson my 5000 btu window model, and works well with a fan.
btw--my plastic tarps do not rattle. tie em so the plastic is tight and no rattle. rattle means the high winds will tear it. torn tarps allow sun and rain inside boat. that is not an option.

Try keeping the sun off the boat first. Just go to Walmart and pick up a couple tarps of various size. Also pick up some of these to secure the tarps down. Run lines where needed to create "structure" for the tarps.

It'll dramatically cool down the boat. The cheap tarps probably won't last more than 1 season, but if they work then you can look into a more long term solution like some proper canvas.

i use plastic tarps in a layered fashion so as to bring into the boat air and prevent su n from baking inside and decking. works great and has worked great for many many years. no , isnt gorgeous, but keeps boat interior 10 degrees cooler than other boats. when it hits 110 in daylight i turn on air conditioner. i only use the lower settingson my 5000 btu window model, and works well with a fan.
btw--my plastic tarps do not rattle. tie em so the plastic is tight and no rattle. rattle means the high winds will tear it. torn tarps allow sun and rain inside boat. that is not an option.

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